Conventionally, there are a large number of light-emitting structures in which nitride semiconductor light-emitting elements such as LEDs (light-emitting diodes) or semiconductor lasers use a sapphire (0001) substrate as a substrate and a plurality of nitride semiconductor layers are formed on the substrate by an epitaxial growth. The nitride semiconductor layer is expressed in a general formula of Al1-x-yGaxInyN (0≦x≦1, 0≦y≦1, 0≦x+y≦1).
As the sapphire (0001) substrate, a slightly inclined substrate having a (0001) surface inclined slightly with an off angle of approximately 0.05° to 0.5° is generally used rather than a non-inclined substrate having a (0001) surface which is not inclined at all with an off angle of zero degree, on the assumption that a surface property or crystallinity of a nitride semiconductor layer to be epitaxially grown is enhanced (for example, see the following Patent Document 1 and Patent Document 2).
The light-emitting element structure has a double heterostructure in which an active layer is interposed between an n-type nitride semiconductor layer and a p-type nitride semiconductor layer. The active layer is constituted by a nitride semiconductor layer having a single quantum well (SQW) structure or a multi quantum well (MQW) structure. In the case in which the active layer is an AlGaN based semiconductor layer, it is possible to regulate a band gap energy within a range, lower and upper limits of which are band gap energies that can be taken by GaN and AlN respectively (approximately 3.4 eV and approximately 6.2 eV) by adjusting an AlN molar fraction (which is also referred to as an AlN composition ratio). Thus, it is possible to obtain an ultraviolet light emitting element having an emission wavelength of approximately 200 nm to 365 nm.